You use electricity to move the car forward. Right?So, if you try to generate back the required electricity, from this "moving forward", you essentially slow the vehicle down to 0.Before you can get even a notable fraction of the used electricity back, the car comes to a stop.Braking can generate electricity, sure. But it is never more electricity than what was used to speed up.Yes, driving downhill yields a net electricity gain. But you can't keep driving downhill forever. Over time it must balance out; ΔUphill = ΔDownhill.﻿

If you want proof, look up any "first law of thermodynamics" experiments. The first law, also known as Law of Conservation of Energy, states that energy cannot be created or destroyed in an isolated system. The second law of thermodynamics states that the entropy of any isolated system always increases.